Haber Process or Haber-Bosch Process

Ammonia from Nitrogen and Hydrogen

Carl Bosch
Portrait of Carl Bosch. Pressephotos/Wikimedia Commons/Public Domain

The Haber process or Haber-Bosch process is the primary industrial method used to make ammonia or fix nitrogen. The Haber process reacts nitrogen and hydrogen gas to form ammonia:

N2 + 3 H2 → 2 NH (ΔH = −92.4 kJ·mol−1)

History of the Haber Process

Fritz Haber, a German chemist, and Robert Le Rossignol, a British chemist, demonstrated the first ammonia synthesis process in 1909. They formed ammonia drop by drop from pressurized air. However, the technology did not exist to extend the pressure required in this tabletop apparatus to commercial production. Carl Bosch, an engineer at BASF, resolved the engineering problems associated with industrial ammonia production. BASF's German Oppau plant commenced ammonia production in 1913.

How the Haber-Bosch Process Works

Haber's original process made ammonia from air. The industrial Haber-Bosch process mixes nitrogen gas and hydrogen gas in a pressure vessel that contains a special catalyst to speed the reaction. From a thermodynamic standpoint, the reaction between nitrogen and hydrogen favors the product at room temperature and pressure, but the reaction does not generate much ammonia. The reaction is exothermic; at increased temperature and atmospheric pressure, the equilibrium quickly switches the other direction. So, the catalyst and increased pressure are the scientific magic behind the process.

Bosch's original catalyst was osmium, but BASF quickly settled upon a less-expensive iron-based catalyst, which is still in use today. Some modern processes employ a ruthenium catalyst, which is more active than the iron catalyst.

Although Bosch originally electrolyzed water to obtain hydrogen, the modern version of the process uses natural gas to obtain methane, which is processed to get hydrogen gas. It is estimated that 3-5% of the world's natural gas production goes toward the Haber process.

The gases pass over the catalyst bed multiple times since conversion to ammonia is only around 15% each time. By the end of the process, about 97% conversion of nitrogen and hydrogen to ammonia is achieved.

Importance of the Haber Process

Some people consider the Haber process to be the most important invention of the past 200 years! The primary reason the Haber process is important is because ammonia is used as a plant fertilizer, enabling farmers to grow enough crops to support an ever-increasing world population. The Haber process supplies 500 million tons (453 billion kilograms) of nitrogen-based fertilizer annually, which is estimated to support food for a third of the people on Earth.

There are negative associations with the Haber process, too. In World War I, the ammonia was used to produce nitric acid to manufacture munitions. Some argue the population explosion, for better or worse, would not have happened without the increased food available because of the fertilizer. Also, the release of nitrogen compounds has had a negative environmental impact.

References

Enriching the Earth: Fritz Haber, Carl Bosch, and the Transformation of World Food Production, Vaclav Smil (2001) ISBN 0-262-19449-X.

US Environmental Protection Agency: Human Alteration of the Global Nitrogen Cycle: Causes and Consequences by Peter M. Vitousek, Chair, John Aber, Robert W. Howarth, Gene E. Likens, Pamela A. Matson, David W. Schindler, William H. Schlesinger, and G. David Tilman

Fritz Haber Biography, Nobel e-Museum, retrieved October 4, 2013.